Hydrogen as a Fuel of the Future

Jules Verne appears to be one of the earliest people to recognize, or at least articulate, the idea of splitting water to produce hydrogen (H2) and oxygen (O2) in order to satisfy the energy requirements of society. As early as 1874 in The Mysterious Island, Jules Verne alluded to clean hydrogen fuels, writing:

Fig. 2. The terawatt renewable energy challenge; the energy mix has to switch from the panel on the left to the panel on the right to cap CO2 levels at safe limits. Data from the International Energy Agency.

"Yes, my friends, I believe that water will someday be employed as fuel, that hydrogen and oxygen, which constitute it, used singly or together, will furnish an inexhaustible source of heat and light____I believe, then, that when the deposits of coal are exhausted, we shall heat and warm ourselves with water. Water will be the coal of the future."

Remarkable words indeed from a prophetic visionary who foresaw also the technological development of spacecraft and submarines. Hydrogen gas was first isolated by Henry Cavendish in 1766 and later recognized as a constituent of water by Lavoisier in 1783.13 The production of hydrogen and oxygen by the electrolytic decomposition of water has been practiced since the year 1800, when the process was first discovered by Nicholson and Carlisle.14 Since then, the idea of society using hydrogen as a primary energy carrier has been explored and refined.

In the late 1920s and the early 1930s a German inventor, Rudolf A. Erren, recognized and worked towards producing hydrogen from off-peak electricity and modifying the internal combustion engine to run on hydrogen.15 Erren's primary objective was to eliminate pollution from the automobile and reduce oil imports. In the 1970s Derek Gregory appears to have been one of the leading advocates in creating the case for a hydrogen-based economy.13,15,16

The literature suggests that the term hydrogen economy may have been coined by H. R. Linden, one of Gregory's colleagues at the Institute of Gas Technology, in 1971.13 Gregory points to hydrogen's environmental benefits and recognizes that, while fossil fuels are inexpensive, requiring the atmosphere to assimilate the byproducts of their combustion is not without consequence.

The water electrolyzer industry grew substantially during the 1920s and 1930s, as elaborated later in Chapter 3. This included products from companies such as Oerli-kon, Norsk Hydro, and Cominco in multi-megawatt sizes.14,17,18 Most of these installations were near hydroelectric plants that supplied an inexpensive source of electricity. As more hydrogen was needed for industries, steam reforming of methane gradually took over as the hydrogen production process of choice because it was less expensive.

Hydrogen is often blamed for the 1937 Hindenburg disaster. The shell of the German airship was a mixture of two major components of rocket fuel, aluminum and iron oxide, and a doping solution which was stretched to waterproof the outer hull. Researchers concluded that the coating of the Hindenburg airship was ignited by an electrical discharge and the ensuing explosion to be inconsistent with a hydrogen fire.19 It turns out that 35 of the 37 people who died in the disaster, perished from jumping or falling from the airship to the ground. Only two of the victims died of burns, and these were from the burning airship coating and on-board diesel fuel.20 Modern laboratory tests confirmed that the 1930s fabric samples to still be combustible.

"Although the benefits of the hydrogen economy are still years away, our biggest challenges from a sustainability standpoint are here today,"

said Mike Nicklas, Past Chair of the American Solar Energy Society, during his opening comments at the first Renewable Hydrogen Forum in Washington, D.C., in April 2003.21

Hydrogen (H) is the simplest of atoms, consisting of one proton and one electron also called a protium. As atoms, hydrogen is very reactive and prefers to join into molecular pairs (H2) and when mixed in sufficient quantities with an oxidant (i.e., air, O2, Cl, F, N2O4, etc.) becomes a combustible mixture. Like all other fuels, H2 requires proper understanding and handling to avoid unwanted flammable or explosive environments. Hydrogen is not a primary source of energy; rather it is an energy carrier much like electricity. Therefore, energy is required to extract hydrogen from substances like natural gas, water, coal, or any other hydrocarbon.

At 25 °C and atmospheric pressure the density of air is 1.225 kg m-3 while hydrogen is 0.0838 kg m-3, making it 14.6 times lighter than air. This is an important safety consideration in that a hydrogen leak will dissipate quickly. Hydrogen's positive buoyancy significantly limits the horizontal spreading of hydrogen that could lead to combustible mixtures. Hydrogen is the lightest (molecular weight 2.016) and smallest of all gases requiring special considerations for containing and sensing a leak.

Figure 3 shows the two types of molecular hydrogen distinguished by the spin, ortho- and para-hydrogen. They differ in the magnetic interactions as ortho-hydrogen atoms are both spinning in the same direction and in para-hydrogen the protons are spinning anti-parallel. At 300 K, the majority (75%) is ortho-hydrogen, while at 20 K 99.8% of the hydrogen molecules are para-hydrogen. As the gas transitions from gas to liquid at 20 K heat is released and ortho-hydrogen becomes unstable.22 Hydrogen becomes a liquid below its boiling point of -253 °C (20 K) at atmospheric pres-

At ambient temperature and pressure hydrogen is colorless, odorless, tasteless and nontoxic. However, leaks of hydrogen (or any gas for that matter) can displace oxygen and act as an asphyxiant. Any atmosphere with less than 19.5% oxygen by volume in considered oxygen deficient and asphyxiation can lead to physiological hazards.

The primary hazard associated with gaseous hydrogen is the unintentional mixing of the fuel with an oxidant (typically air) in the presence of an ignition source. Hydrogen fires and deflagrations have resulted when concentrations within the flamma-bility limit were ignited by seemingly harmless ignition sources. Ignition sources include electrical, mechanical, thermal and chemical. For example; sparks from valves, electrostatic discharges, sparks from electrical equipment, mechanical impact, welding and cutting, open flame, personnel smoking, catalyst particles and lightning strikes in the proximity of hydrogen vent stacks.23

With the exception of helium, hydrogen has the lowest boiling point at atmospheric pressure of where it becomes a transparent and odorless liquid. Liquid hydrogen has a specific gravity of 0.071, which is roughly 1/14th the density of water and is neither corrosive nor reactive. The low specific gravity of liquid hydrogen further reveals hydrogen's low volumetric energy density in that a cubic meter of water contains more hydrogen (111 kg) than a cubic meter of pure hydrogen in liquid state (71 kg). The values of the main physical properties of gaseous hydrogen are shown in Table 1.

Leaking hydrogen gas and (once ignited) its flame are nearly invisible. The pale blue flame of a hydrogen fire is barely visible and is often detected by placing a standard household wicker broom in the path of the suspected hydrogen flame. The hydrogen flame temperature in air (2045 C, 3713 F) releases most of its energy in the ultraviolet (UV) region requiring UV sensors for detecting the presence of a flare or fire. The UV radiation from a flaring hydrogen fire can also cause burns akin to over-exposure to the sun's damaging UV radiation.

Table 1. Selected properties of gaseous hydrogen at 20 °C and 1 atm.

Physical Property

Units

Molecular weight

2.016

Density

0.0838

kg/m3

Specific gravity

0.0696

(Air = 1)

Viscosity

8.813 x 10-5

g/cm sec

Diffusivity

1.697

m2/hr

Thermal conductivity

0.1825

W/m K

Expansion ratio

1:848

Liquid to gas

Boiling point (1 atm)

-253(-423)

°C (°F)

Specific heat, constant pressure

14.29

J/g K

Specific heat, constant volume

10.16

J/g K

Specific volume

11.93

m3/kg

Diffusion coefficient in air

6.10

cm2/sec

Enthalpy

4098

kJ/kg

Entropy

64.44

J/g K

The amount of thermal radiation (heat) emitted from a hydrogen flame is low and is hard to detect by feeling (low emissivity). Most commercially available combustible gas detectors can be calibrated for hydrogen detection. Typically alarms from these sensors are set by the manufacturer between 10%-50% of the lower flammabil-ity limit (LFL) of hydrogen to avoid the presence of an unwanted flammable environment.

Table 2 compares the same fuels as above and reports their volumetric energy density in kg m-3. Hydrogen has the highest energy content per unit mass than any fuel making it especially valuable when traveling into space. As mentioned earlier, hydrogen suffers volumetrically when compared with traditional fuels making storing sufficient on-board terrestrial vehicles an engineering challenge.

The LFL of hydrogen represents the minimum concentration required below which the mixture is too lean to support combustion.24 Hydrogen has a wide flam-mability range of (4%-75%) while gasoline is (1.5%—7%) when mixed with air at standard temperature (25 °C) and pressure (1 atm). Hydrogen in oxygen has a slightly wider flammability range (4%-95%). Table 3 summarizes a selected number of important combustion properties of hydrogen.

Table 2. Comparing hydrogen properties with other fuels. Based on LHV and 1 atm,

Table 2. Comparing hydrogen properties with other fuels. Based on LHV and 1 atm,

Each fuel is limited to a fixed amount of energy it can release when it reacts with an oxidant. Every fuel has been experimentally tested to determine the amount of energy it can release and is reported as the fuel's higher heating value (HHV) and lower heating value (LHV). The difference between the two values is the latent heat of vaporization of water, and the LHV assumes this energy is not recovered.22 In other words, LHVs neglect the energy in the water vapor formed by the combustion of hydrogen in the fuel because it may be impractical to recover the energy released when water condenses. This heat of vaporization typically represents about 10% of the energy content.

It is often confusing to know which heating value to use when dealing with similar processes such as electrolysis and fuel cells. The appropriate heating value depends on the phase of the water in the reaction products. When water is in liquid form, the HHV is used; if water vapor (or steam) is formed in the reaction, then the LHV would be appropriate. An important distinction is that water is produced in the form of vapor in a fuel cell as well as in a combustion reaction and, therefore, the LHV represents the amount of energy available to do work. Table 4 shows both the LHV and the HHV for common fuels.

Obviously, the most important virtue of using hydrogen as a fuel is its pollutionfree nature. When burned in air, the main combustion product is water with O2 in a fuel cell to directly produce electricity; the only emission is water vapor. Indeed this

Table 4. HHVs and LHVs at 25 °C and 1 atm of common fuels, kJ g-1 a

Fuel

HHV

LHV

Hydrogen

141.9

119.9

Methane

55.5

50.0

Gasoline

47.5

44.5

Diesel

44.8

42.5

Methanol

20.0

Fig. 4. Decarbonization of the energy source over the centuries.

fuel cell product is clean enough to furnish drinking water to the crews in spacecraft! Crucially, the use of hydrogen completes the decarbonization trend that has accompanied the evolution of energy sources for mankind over the centuries (Figure 4). The combustion of H2, unlike fossil fuels, generates no CO2. Unlike fossil fuels, however, hydrogen is not an energy source but is an energy carrier since it almost never occurs by itself in nature, at least terrestrially. (The atmospheres of other planets, e.g., Mars, are rich in hydrogen. Should space travel prove to be economical and accessible in the future, we may have a viable means to "mine" H2 as we are doing for petroleum and coal these days!)

In the interim timeframe: Where is the H2 to come from? Historically, H2 has been used for energy since the 1800s. It is a major constituent (up to ~50% by volume) of syngas generated from the gasification of coal, wood, or municipal wastes. Indeed, syngas was used in urban homes in the U. S. for heating and cooking purposes from the mid-1800s until the 1940s and is still used in parts of Europe, Latin America and China where natural gas is unavailable or too expensive. Most of the H2 manufactured these days comes from the steam reforming of methane (see above). Other processes for making H2 from fossil fuel sources include the water gas shift reactions. Neither of these approaches is carbon-neutral in that significant amounts of CO2 are generated in the H2 manufacture process itself.

The ultimate goal would be to produce H2 with little or no greenhouse gas emissions. One option is to combine H2 production from fossil fuels with CO2 sequestration. Carbon sequestration, however, is as yet an unproven technology. Another approach is biomass gasification—heating organic materials such as wood and crop wastes so that they release H2 and carbon monoxide. This technique is carbonneutral because any carbon emissions are offset by the CO2 absorbed by the plants during their growth. A third possibility is the electrolysis of water using power generated by renewable energy sources such as wind turbines and solar cells. This approach is discussed in Chapters 3 and 4.

Although electrolysis and biomass gasification involve no major technical hurdles, they are cost-prohibitive, at least at present: $6-10 per kilogram of H2 pro-

Fig. 5. The water splitting/hydrogen fuel cycle without (left panel) or with (right panel) inclusion of solar energy input.

duced.The goal is to be able to develop and scale-up technologies to afford a pump price for H2 of $2-4 per kilogram. In such a scenario, hydrogen in a fuel cell powered car would cost less per kilometer than gasoline in a conventional car today.

Clearly, water would be the ideal and most sustainable source for H2 and this H2 generation concept dates back two centuries. Table 5 summarizes various schemes for generating H2 via splitting of water and Figure 5 depicts the water splitting/hydrogen fuel cycle without (left panel) or with (right panel) inclusion of solar energy input. The approaches considered in Table 5 and Figure 5 form the topics of discussion in Chapters 4 through 7 of this book.

The power needed for water electrolysis could come from nuclear energy although producing H2 this way would not be significantly cheaper than using renewable power sources. Nuclear plants can generate H2 in a non-electrolytic, thermal mode because of the intense heat generated in a thermonuclear reaction. This ap-

Table 5. The ability of nuclear and various renewable energy sources to meet the 14-20 TW demand of carbon-free power by 2050.a

Source

Power available TW

Comments

Biomass

l-10

Entire arable land mass of the planet must be used excluding the area needed to house 9 billion people

proach, while potentially cost-effective, has not been demonstrated yet. It must be noted that any option involving nuclear power has the same hurdles that have dogged the nuclear electric power industry for decades, namely those of waste disposal problems, proliferation concerns and lack of public acceptance. (This contrasts with the success of the nuclear power industry in some countries, e.g., France.) Producing 10 TW of nuclear power would require the construction of a new 1-GWe nuclear fission plant somewhere in the world every other day for the next 50 years!25

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